New Jupiter Scars May Be Only Skin Deep

By MALCOLM W. BROWNE

Published: July 20, 1994

As more cometary fragments hurtled into Jupiter yesterday, black, Earth-size spots blossoming across the planet's southern hemisphere left it looking battered. But some astronomers believe that the havoc, evident even to amateur stargazers with back-yard telescopes, may be only skin deep.

Scientists began to catch their breath yesterday after three days of continuous excitement, during which the face of Jupiter has changed from minute to minute in a planetary show unequaled in recorded history. Jupiter's bombardment by fragments of Comet Shoemaker-Levy 9, which began Saturday afternoon, will continue until Friday, when the last of 21 large fragments is expected to hit.

Observatories throughout the world are scrambling to measure and record each impact, and there has been little time to begin analyzing the mountains of data they have accumulated. But the first hints of a bountiful harvest of scientific information have begun to appear.

For example, astronomers hope for the first time to be able to analyze the chemical contents of a comet's nucleus. Matter From Dying Comet

Using one of the large telescopes of the National Optical Astronomy Observatory atop Kitt Peak in Arizona, Dr. Marcia J. Rieke and her husband, Dr. George H. Rieke, both astronomy professors at the University of Arizona, have seen evidence that the huge fiery plumes arising from impact sites may consist mostly of matter from the dying comet, rather than material from the depths of Jupiter.

The Riekes reported yesterday that a spectral analysis of infrared radiation emitted by the fiery plume from Fragment A, the first in the series, shows no sign of the methane, ammonia or water ice present in Jupiter's hydrogen atmosphere. Instead, they said, the plume seems to consist mainly of dirt from the comet's nucleus.

If this conclusion is borne out by more observations it would tend to support the view of Dr. Clark Chapman, an astronomer with the Planetary Science Institute in Tucson, Ariz. Dr. Clifford says he believes that the explosions are occurring at high altitudes in Jupiter's atmosphere and may not significantly affect the dense lower atmosphere.

Part of the evidence for this, Dr. Chapman said, is that flashes of light from the impacts show no evidence that methane gas has strongly absorbed certain wavelengths of light. If the explosions were occurring at great depths, he said, it could be expected that the methane in Jupiter's atmosphere would absorb and block light in those wavelengths. Deep Penetration Seen

A contrasting view was expressed by Dr. Heidi Hammel of the Massachusetts Institute of Technology, a team leader of observers using the Hubble Space Telescope. Dr. Hammel believes the impacts penetrate deeply into Jupiter's atmosphere.

Scientists do not know precisely how deep Jupiter's atmosphere is, because the planet is shrouded by dense clouds. But they believe it is about 11,000 miles deep, extending to a depth about one-fourth of the distance to the center of the planet.

At that point the pressure of the atmosphere is so enormous that hydrogen is crushed to a solid, metallic form, which constitutes Jupiter's inner shell. Deep within this shell is believed to be a small, rocky core about the size of Earth.

If the cometary impacts on Jupiter cause only shallow disruptions of its atmosphere, the huge blotches the impacts have left may fade and disappear fairly soon. If the explosions occurred at great depths, the spots could survive for hundreds of years.

"We can't be dogmatic about this," Dr. Chapman said. "Never in recorded history can I find evidence that planetary features as conspicuous as those we're now seeing have appeared before. Not even Jupiter's Great Red Spot, which we know has been present for at least 300 years, is as prominent as the big, dark spot left by the impact of Fragment G on Monday morning."

The Great Red Spot, a raging storm system in Jupiter's southern hemisphere, is as big as Earth. Watching the Show at Home

Dr. Chapman and his team of, astronomers use the huge telescopes on Kitt Peak for their professional observations. Jupiter and Comet Shoemaker-Levy 9 have been putting on such a good show that Dr. Chapman decided to watch at home as well as at the observatory.

"I got out a 10-inch-diameter telescope my parents gave me when I was in high school and looked at those fascinating new spots on Jupiter," he said. "Anyone with a telescope should be out looking while this goes on."

Meanwhile, observers at the big telescopes were also enjoying the fireworks. The European Southern Observatory at Cerro La Silla, Chile, one of the world's main assemblages of astronomical observatories, reported that the impact of Fragment H on Monday afternoon caused a flash 50 times brighter than the normal brightness of Jupiter itself.

And the South Pole Infrared Explorer telescope at the South Pole reported that Fragment H produced a flash and splash comparable to those of Fragment G, which had been regarded as the largest of the whole series. Some Disappointments, Too

But there were a few disappointments, too.

Since none of the impacts occur on the side of Jupiter that can be seen from Earth, astronomers looking for flashes have been trying to see them as light reflected from Jupiter's moons.

Many pinned their hopes on the impact of Fragment K yesterday morning, the only fragment that was expected to hit when Europa, one of Jupiter's moons, was eclipsed by the planet. They hoped that the moon, shaded from sunlight for a change, would serve as a reflective screen for the flash from Fragment K.

But neither the South Pole telescope nor astronomers from the University of Maryland at the Perth Observatory in Australia observed any reflected flash from Europa. "It's very puzzling," one astronomer said.

Photo: A sequence of images from the European Southern Observatory in Chile shows the a plume developing where a comet fragment hit Jupiter. (Associated Press)